The aims of this study were, firstly, to study the association between parents’ and teachers’ ratings for the Finnish version of the Autism Spectrum Screening Questionnaire (ASSQ), secondly, to find out whether the original cut-off scores of the ASSQ identify primary school-aged children with Asperger syndrome (AS) or autism by using the Finnish ASSQ, and thirdly, to evaluate the validity of the ASSQ. Parents and/or teachers of higher-functioning (full-scale intelligence quotient ≥ 50) 8-year-old total population school children (n = 4,408) and 7–12-year-old outpatients with AS/autism (n = 47) completed the Finnish version of the ASSQ. Agreement between informants was slight. In the whole total population, low positive correlation was found between parents’ and teachers’ ratings, while in the sample of high-scoring children the correlation turned out to be negative. A cut-off of 30 for parents’ and teacher's summed score and 22 for teachers’ single score is recommended. A valid cut-off for parents’ single score could not been estimated. The clinicians are reminded that the ASSQ is a screening instrument, not a diagnosing instrument. The importance of using both parents’ and teachers’ ratings for screening in clinical settings is underlined.